Initial complexes had a left bundle branch block (LBBB) morphology but after the PVC or premature ventricular beat the QRS duration is normal.
Retograde Concealed Penetration
The rate during the LBBB and the normal QRS complex is the same (~88 bpm) and the PRI's are the same (~280 ms).
This aberration may be due to repetitive (concealed) transeptal penetration from the impulse coming from the right bundle perpetuating local refractoriness or repititive impulse collision.
Peel back refractoriness
The abrupt recovery was due to the PVC that excited the left bundle bracnh early and allowed more time for it to recover ("peel back refractoriness") and conduct normally after the next supraventricular impulse (resulting in normal QRS).
Fisch C., Zipes DP and McHenry PL. 1973. Rate Deependent Aberancy. Circ 48:714-724 (http://circ.ahajournals.org/content/48/4/714.ful l.pdf+html)
Fisch C and Knoebel SB. 2000. Electrocardiography of Clinical Arrhythmia. New York. Futura Publishing Co.
Neiger JS and Trohman RG. Differential Diagnosis of Tachycardia with a Typical LBBB Morphology. WJC 3(5):127-134
Wang SH et al.2001. Electrophysiologic Characteristics of Ventricular
Extrastimulation-Induced Dissipation of Functional Bundle
Branch Block Associated with Supraventricular Tachycardia. J Cardiovasc Electrophysiol, Vol. 12, pp. 928-934